A tangent is a topic nearly unrelated to the main topic, but having a point in common with it. (Wiktionary)
So, Tangental Thinking is when a person thinks their nearly unrelated point is very much related to the topic, or proposition. It has a point in common, but it misses the point. They are unaware that the points they raise in a discussion are indeed tangents.
This thinking is beginning (if it hasn’t already) to define Christian thinking in the last days. It’s rare to meet a Christian who doesn’t think tangentally.
One reason is that the religious system, by virtue of it’s spectator/audience quality, has not trained Christians to think in a sound, logical and coherent manner. When we are “the audience” then we don’t articulate the truth. It’s by articulating the truth of God’s Word that our mind is trained, reversing the effects of sin, which caused us not to have a sound mind.
Ian Vincent 
August 28, 2010 at 12:25 am
Now, there’s nothing wrong at all about going off on a tangent. The point is, if one does go off on a tangent, they should be aware THAT IT IS INDEED A TANGENT, and not think they are addressing the issue.
Like, to say: OK, this is a tangent, it doesn’t address the issue.
The PROBLEM with tangental thinking is that people THINK they are addressing/discussing an issue when they are not.
This could be due to a lack of soundness in their thinking.
It could also be dishonesty. Instead of honestly admitting they don’t know an answer, or that they don’t want to address an issue, they keep face by answering tangentally. This way they have avoided the issue but still kept face.